The proposed research will determine the presence and functional ability of dopamine receptor agonists to inhibit postganglionic sympathetic neuronal acitivty in various species of laboratoty animals and humans. Our studies evaluating the ability of dopamine receptor agonists and clonidine to inhibit adrenergic transmission to atria of laboratory animals reveal a wide species variation. Clonidine (alpha2-receptor agonist) is effective in guinea-pigs, rabbits, rats, dogs and partial agonist activity in cats. Apomorphine (dopamine receptor agonist) is inactive in atria of guinea-pigs and rats, active in rabbits, cats and dogs. We will evaluate clonidine and a series of dopamine receptor agonists for ability to inhibit adrenergic transmission to saphenous veins and mesenteric arteries. We will determine the relative effectiveness of clondine and the dopamine receptor agonists to inhibit adrenergic transmission to blood vessels. We will compare these results to activity in atria. We will determine species sensitivity and selectivity for alpha2 and dopamine receptor inhibition of transmission. The results will indicate similarily and differences within a species and between species. The results will be important for the following reasons: (1) The presence or absence of dopamine receptors on the human adrenergic nerve terminal has not been determined (2) information concerning which animals species is most similar ot humans is needed for evaluation of dopamine receptor agonists as possible anti-hypertensive agents. New long acting synthehtic agents are now known and the animal species most similar to humans must be determined for proper evaluation of these agents, ie. spontaneous hypertenive rats may not be the proper species for evaluation of dopamine receptor agonists.